Field of the Invention
This invention relates to a focus adjusting device for flange back adjustment of an inner focus lens assembly having a compensator lens.
Description of the Prior Art
In an inner focus lens assembly whose front lens is fixed, when a zoom lens is moved, focal position for a focus lens varies with movement of the zoom lens. To cope with the variation, in existing video cameras using such inner focus lens assemblies, information on changes in focal position for the focus lens with movements of the zoom lens is stored in memory as mapping data. A curve indicating changes in focal position of the focus lens with movements of the zoom lens is called a tracking curve. When the zoom lens is moved, position of the focus lens is controlled in accordance with the data in the form of the tracking curve stored in the memory. Thereby, even when the zoom lens is moved, focalization is maintained. Such control is called zoom tracking control.
In existing video cameras equipped with such an inner focus lens assembly as described above, a tracking curve is stored as mapping data in memory to control position of the focus lens in accordance with the tracking curve when the zoom lens is moved. Although the tracking curve is previously prepared, a difference occurs between an actual tracking curve and the designed tracking curve stored as mapping data, because of a lens mounting error or the like. For correcting the difference, flange back adjustment is executed in existing video cameras having an inner focus lens assembly.
FIGS. 1 and 2 are views for illustrating flange back adjustment in an existing video camera which does not include a compensator lens. FIG. 1 is a flow chart of existing flange back adjustment, and FIG. 2 illustrates movements of the lenses in respective steps. In FIG. 2, respective reference numerals correspond to movements of the lenses in respective steps. Still in FIG. 2, the abscissa indicates position of the zoom lens, and ordinate indicates position of the focus lens. In FIG. 2, reference numeral All refers to a tracking curve stored as mapping data, B11 refers to an actual tracking curve.
As shown in FIGS. 1 and 2, the zoom lens is first set at a position corresponding to a point of inflexion as designed (step ST101). The point of inflexion corresponds to a peak value of the tracking curve.
When the zoom lens is brought to the designed point of inflexion, the focus lens is controlled to move to a focal position. When the focus lens reaches the focal position, the present focal position is stored (step ST102).
After the focus lens reaches the focal position, it is moved by a predetermined amount T. The amount T is a designed value for movement of the focus lens from the point of inflexion to the telephoto end (step ST103).
When the focus lens reaches the telephoto end, the zoom lens is moved to a focal position, and the present focal position is stored as the telephoto end (step ST104).
The zoom lens is moved by a predetermined amount S. The amount S is a designed value for movement of the zoom lens from the point of inflexion to the telephoto end (step ST105).
When the zoom lens is moved, the focus lens is controlled to move to a focal position, and the present focal position is stored (step ST106).
The previous focal position for the focus lens stored in step ST102 is compared with the present focal position to determine whether a difference between the positions for the focus lens is within a predetermined value (step ST107).
If the difference between the previous and present focal positions for the focus lens does not reach the predetermined value, control returns to step ST103, and the same process is repeated until difference between former and present focal positions for the focus lens comes within the predetermined value.
When a difference between former and present focal positions for the focus lens comes within the predetermined value, the focus lens is moved from a wide end to the telephoto end by a designed moving amount for the focus lens (step ST108).
The zoom lens is moved to a focal position, and the position is stored as a wide end (step ST109).
Inner focus lens assemblies used in existing video cameras include a fixed front lens, a zoom lens, and a focus lens, but do not include a compensator lens. In inner focus lens assemblies including a compensator lens, a difference from a focal position occurring upon moving its zoom lens is compensated by moving the compensator lens by means of a mechanical cam.
In existing inner focus lens assemblies which do not include a compensator lens, the tracking curve has an essentially chevron like configuration as shown in FIG. 2, and includes a sharp curved portion. That is to say, the curve from the point of inflexion to the telephoto end is very sharp. Therefore, when the zoom lens is moved at a high speed, the focus lens cannot follow it. At the same time, in sharply curved position of the tracking curve, a focus error exceeds the depth of field unless positional accuracy of the focus lens is increased. As a result, it is still difficult to move the zoom lens at a high speed. Further, since the slope of the tracking curve becomes sharper as the magnification increases, lenses of high magnification cannot be used.
In contrast, in inner focus lens assemblies including a compensator lens, a difference from a focal position which occurs upon moving a zoom lens is compensated by moving the compensator lens through a mechanical cam, and the tracking curve is relatively flat as shown in FIG. 2. Therefore, even when the zoom lens is moved at a high speed, the focus lens can follow it. In addition, even when positional accuracy of the focus lens is not so high, it is easy to maintain the focal position within the depth of field.
In inner focus lens assemblies including a compensator lens, unlike the existing inner focus lenses which do not use a compensator lens, tracking curves do not include a point of inflexion, and flange back adjustment utilizing a point of inflexion is impossible. Moreover, unlike the existing inner focus lens assemblies not including a compensator lens, tracking curves are very moderate at their wide ends. Therefore, it is very difficult to find a focal position by moving the zoom lens on the wide end side.